Main Eco Compact Common Problems & Fault Codes (2026 Guide)

The 6 most common faults

118 High DIY-safe

Your boiler has stopped working because the water pressure inside the heating system has dropped too low to operate safely.

What to try first

Locate the external filling loop, which is a flexible braided hose under the boiler connected to two valves.Open the valves slowly to allow water into the system until the pressure gauge on the front of the boiler reaches between 1.0 and 1.5 bar.Ensure both valves are fully closed and check the pressure gauge to confirm it is stable.

Est. cost: £80-150
Parts: Filling loop, Pressure relief valve, Pressure sensor

Full guide for 118

20 High Engineer

Your boiler has detected an internal issue with its safety controls, causing it to stop operating to ensure your home remains safe.

Est. cost: £150-300
Parts: Printed Circuit Board (PCB), Wiring harness, Gas valve

Full guide for 20

28 High Engineer

Your boiler has detected an issue with the sensor that reads the temperature of the water flowing out of the unit, meaning it has safely shut down to prevent damage.

Est. cost: £150-250
Parts: Flow temperature sensor, Wiring harness

Full guide for 28

40 High Engineer

Your boiler has detected an internal issue with its electrical controls or sensors, causing the system to stop working reliably.

Est. cost: £150-300
Parts: Printed Circuit Board (PCB), Wiring harness, Temperature sensor

Full guide for 40

103 High Engineer

Your boiler is struggling to communicate with an optional safety monitoring device, which is causing it to suspend operation as a precaution.

Est. cost: £120-250
Parts: Safety Assist module, PCB interface cable

Full guide for 103

104 High Engineer

Your boiler is having trouble communicating with an optional safety device, which is causing it to stop operating for your protection.

Est. cost: £120-250
Parts: Safety Assist module, Wiring harness, PCB

Full guide for 104

Parts most often replaced

on the Main Eco Compact

Filling loopPressure relief valvePressure sensorPrinted Circuit Board (PCB)Wiring harnessGas valve

Based on parts cited in our fault code database. Your engineer will confirm what's actually needed after diagnosis.

Call a Gas Safe engineer if…

You can smell gas or see signs of a leak
The Main shows an Emergency or High severity code
The boiler keeps locking out after repeated resets
You've tried the DIY checks and the fault hasn't cleared
There's visible water leaking from the boiler
The flame is yellow or orange instead of blue

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See the full 20-code list for the Main Eco Compact